extern int fork(void);
extern int exit(void);
extern int wait(void);
extern int kill(int);
extern int getpid(void);
extern int write(int, const void*, int);
extern int read(int, void*, int);
extern int sbrk(int);

int strlen(const char *s) {
    int n = 0;
    while (s && s[n]) n++;
    return n;
}